Abstract
We propose $ømega$MSO$\Join$BAPA, an expressive logic for
describing countable structures, which subsumes and
transcends both Counting Monadic Second-Order Logic (CMSO)
and Boolean Algebra with Presburger Arithmetic (BAPA). We
show that satisfiability of $ømega$MSO$\Join$BAPA is
decidable over the class of labeled infinite binary trees,
whereas it becomes undecidable even for a rather mild
relaxations. The decidability result is established by an
elaborate multi-step transformation into a particular normal
form, followed by the deployment of Parikh-Muller Tree
Automata, a novel kind of automaton for infinite labeled
binary trees, integrating and generalizing both Muller and
Parikh automata while still exhibiting a decidable (in fact
PSpace-complete) emptiness problem. By means of
MSO-interpretations, we lift the decidability result to all
tree-interpretable classes of structures, including the
classes of finite/countable structures of bounded
treewidth/cliquewidth/partitionwidth. We generalize the
result further by showing that decidability is even preserved
when coupling width-restricted $ømega$MSO$\Join$BAPA with
width-unrestricted two-variable logic with advanced counting.
A final showcase demonstrates how our results can be
leveraged to harvest decidability results for expressive
$\mu$-calculi extended by global Presburger constraints.
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